Aquinas did not think the finite human mind could know what God is directly, therefore God's existence is not self-evident to us.[2] In other words, he rejected Anselm's ontological argument. So instead we must infer God's existence indirectly, from his effects which are more known to us.[3]

In the world we can see that at least some things are changing. Whatever is changing is being changed by something else. If that by which it is changing is itself changed, then it too is being changed by something else. But this chain cannot be infinitely long, so there must be something that causes change without itself changing. This everyone understands to be God.[5][6]

Aquinas uses the term "motion" in his argument, but by this he understands any kind of change, and more specifically a transit from potentiality to actuality.[7] Since a potential does not yet exist, it cannot cause itself to exist and can therefore only be brought into existence by something already existing.[1] When Aquinas argues that a causal chain cannot be infinitely long, he does not have in mind a chain where each element is a prior event that causes the next event; in other words, he is not arguing for a first event in a sequence. Rather, his argument is that a chain of concurrent effects must be rooted ultimately in a cause capable of generating these effects, and hence for a cause that is first in the hierarchical sense, not the temporal sense.[8] His thinking here relies on what would later be labelled "essentially ordered causal series" by John Duns Scotus.[9] This is a causal series in which the immediately observable elements are not capable of generating the effect in question, and a cause capable of doing so is inferred at the far end of the chain. In other words, he rejected the argument that the universe must have had a beginning.[10] Finally, his concept of God has minimal content by the end of the argument, which he fleshes out through the rest of the Summa Theologica. For example, the question of whether "God" has a body or is composed of matter is answered in question three, immediately following the Five Ways.[11]

In the world we can see that things are caused. But it is not possible for something to be the cause of itself, because this would entail that it exists prior to itself, which is a contradiction. If that by which it is caused is itself caused, then it too must have a cause. But this cannot be an infinitely long chain, so therefore there must be a cause which is not itself caused by anything further. This everyone understands to be God.[6]

As in the First Way, the causes Aquinas has in mind are not sequential events, but rather simultaneously existing dependency relationships. For example, plant growth depends on sunlight, which depends on gravity, which depends on mass.[8] Aquinas is not arguing for a cause that is first in a sequence, but rather first in a hierarchy: a principal cause, rather than a derivative cause.[12]

In the world we see things that are possible to be and possible not to be. In other words, perishable things. But if everything were contingent and thus capable of going out of existence, then, given infinite time, this possibility would be realized and nothing would exist now. But things clearly do exist now. Therefore, there must be something that is imperishable: a necessary being. This everyone understands to be God.[5][6]

The argument begins with the observation that things around us come into and go out of existence: animals die, buildings are destroyed, etc. But if everything were like this, then, assuming an infinite past, all possibilities would be realized and everything would go out of existence. Since this is clearly not the case, then there must be at least one thing that does not have the possibility of going out of existence.[6]

We see things in the world that vary in degrees of goodness, truth, nobility, etc. For example, sick animals and healthy animals, and well drawn circles as well as poorly drawn ones. But judging something as being "more" or "less" implies some standard against which it is being judged. Therefore, there is something which is goodness itself, and this everyone understands to be God.[6]

The argument is rooted in Aristotle and Plato but its developed form is found in Anselm's Monologion.[13][14] Although the argument has Platonic influences, Aquinas was not a Platonist and did not believe in the Theory of Forms. Rather, he is arguing that things that only have partial or flawed existence indicate that they are not their own sources of existence, and so must rely on something else as the source of their existence.[15] The argument makes use of the theory of transcendentals: properties of existence. For example, "true" presents an aspect of existence, as any existent thing will be "true" insofar as it is true that it exists. Or "one," insofar as any existent thing will be (at least) "one thing."[16]

We see various non-intelligent objects in the world behaving in regular ways. This cannot be due to chance, since then they would not behave with predictable results. So their behavior must be set. But it cannot be set by themselves, since they are non-intelligent and have no notion of how to set behavior. Therefore, their behavior must be set by something else, and by implication something that must be intelligent. This everyone understands to be God.[6]

The Fifth Way depends on the concept of Aristotle's final causes. Aristotle argued that a complete explanation of an object will involve knowledge of how it came to be (efficient cause), what material it consists of (material cause), how that material is structured (formal cause), and the specific behaviors associated with the type of thing it is (final cause).[17] The concept of final causes involves the concept of dispositions or "ends": a specific goal or aim towards which something strives. For example, acorns regularly develop into oak trees but never into sea lions. The oak tree is the "end" towards which the acorn "points," its disposition, even if it fails to achieve maturity. The aims and goals of intelligent beings is easily explained by the fact that they consciously set those goals for themselves. The implication is that if something has a goal or end towards which it strives, it is either because it is intelligent or because something intelligent is guiding it.[18] It must be emphasized that this argument is distinct from the design argument associated with William Paley and the Intelligent Design movement. The latter implicitly argue that objects in the world do not have inherent dispositions or ends, but, like Paley's watch, will not naturally have a purpose unless forced to do some outside agency.[18] The latter also focus on complexity and interworking parts as the effect needing explanation, whereas the Fifth Way takes as its starting point any regularity.[18]

Kant argued that our minds give structure to the raw materials of reality, and that the world is therefore divided into the phenomenal world (the world we experience and know), and the noumenal world (the world as it is "in itself," which we can never know).[20] Since the cosmological arguments reason from what we experience, and hence the phenomenal world, to an inferred cause, and hence the noumenal world, since the noumenal world lies beyond our knowledge we can never know what's there.[21] Kant also argued that the concept of a necessary being is incoherent, and that the cosmological argument presupposes its coherence, and hence the arguments fail.[22]

Hume argued that since we can conceive of causes and effects as separate, there is no necessary connection between them and therefore we cannot necessarily reason from an observed effect to an inferred cause.[23] Hume also argued that explaining the causes of individual elements explains everything, and therefore there is no need for a cause of the whole of reality.[24][25]

The 20th-century philosopher of religion Richard Swinburne argued in his book, Simplicity as Evidence of Truth, that these arguments are only strong when collected together, and that individually each of them is weak.[26]

Philosopher Keith Ward claims in his book Why there almost certainly is a God: Doubting Dawkins that Dawkins mis-stated the five ways, and thus responds with a straw man. Ward defended the utility of the five ways (for instance, on the fourth argument he states that all possible smells must pre-exist in the mind of God, but that God, being by his nature non-physical, does not himself stink) whilst pointing out that they only constitute a proof of God if one first begins with a proposition that the universe can be rationally understood. Nevertheless, he argues that they are useful in allowing us to understand what God will be like given this initial presupposition.[27]

More recently the prominent Thomistic philosopher Edward Feser has argued in his book Aquinas: A Beginner’s Guide that Dawkins, Hume, Kant, and most modern philosophers do not have a correct understanding of Aquinas at all; that the arguments are often difficult to translate into modern terms.[28]

Eastern Orthodox theologian David Bentley Hart says that Dawkins "devoted several pages of The God Delusion to a discussion of the 'Five Ways' of Thomas Aquinas but never thought to avail himself of the services of some scholar of ancient and medieval thought who might have explained them to him ... As a result, he not only mistook the Five Ways for Thomas's comprehensive statement on why we should believe in God, which they most definitely are not, but ended up completely misrepresenting the logic of every single one of them, and at the most basic levels."[29] Hart said of Dawkins treatment of Aquinas' arguments that:

Not knowing the scholastic distinction between primary and secondary causality, for instance, [Dawkins] imagined that Thomas's talk of a "first cause" referred to the initial temporal causal agency in a continuous temporal series of discrete causes. He thought that Thomas's logic requires the universe to have had a temporal beginning, which Thomas explicitly and repeatedly made clear is not the case. He anachronistically mistook Thomas's argument from universal natural teleology for an argument from apparent "Intelligent Design" in nature. He thought Thomas's proof from universal "motion" concerned only physical movement in space, "local motion," rather than the ontological movement from potency to act. He mistook Thomas's argument from degrees of transcendental perfection for an argument from degrees of quantitative magnitude, which by definition have no perfect sum. (Admittedly, those last two are a bit difficult for modern persons, but he might have asked all the same.)"[29]